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Shared, Derived Characteristics of Birds B. Archaeopteryx: Significance, History, As Bird, As Dinosaur C

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Shared, Derived Characteristics of Birds B. Archaeopteryx: Significance, History, As Bird, As Dinosaur C THE AVIALAE A. What is a bird? Shared, derived characteristics of birds B. Archaeopteryx: significance, history, as bird, as dinosaur C. Mesozoic birds D. Feathered dinosaurs E. Origin of birds Hypothesis1: birds are not dinosaur descendents - evidence Hypothesis 2: birds ARE dinosaur descendents - evidence A. What is a bird? Living birds are diverse (~9,000 species) and they possess some derived characters that are analogous to significant derived characteristics in living mammals (i.e., they evolved independently in birds and mammals). These include upright posture, high metabolic rate/warm bloodedness and large brain size. All birds through time (beginning with the Late Jurassic Archaeopteryx) belong to the clade Avialae; modern (Cenozoic) birds belong to the clade Aves, also called Neornithes in some of the images from the lecture. Shared derived traits that distinguish Aves from other living vertebrates Feathers used to be considered the essential defining character of birds, but feathered dinosaur fossils found in China in the last decade show that that is not the case; birds are now defined by the suite of physical, physiological and skeletal specializations that apparently evolved to enhance their ability to fly. They include (but are not limited to): Feathers: planar feathers with asymmetrically-positioned shafts used for flight (aka the ‘primaries’ on living birds). Feathers come in a variety of forms, including down, contour feathers, semi-plumes, symmetrical rectrices (tail feathers) and remiges (arm feathers). Feather have the following components: quill, shaft, barbs and barbules, but not all feathers contain all of these elements. Feather architecture varies with function. Endotherm (warm-bloodedness): necessary for prolonged flight Four-chambered heart: necessary for endothermy (as in mammals) Expanded skulls with huge orbits: large brains and eyes that can sense ultraviolet light. Generally much better vision and color perception than human eyes. Heads supported by long, mobile necks with very complex muscle control. 1 Large brains: facilitate cogitation and sensory perception, including vision (see above), exceptional hearing abilities (especially for long- wavelength sound), and atmospheric pressure detection. Toothlessness: modern birds have no teeth Carpometacarpus: reduced, fused wrist and hand, supports wingtips and to reduces flexion Alula: feather on tip of hand that breaks turbulence over wing. Shoulder joint: birds have a unique ‘pulley’ arrangement in their shoulders in which a tendon attached to the supracoracoideus muscle passes through a hole formed by the intersection of the coracoid, furcula and scapula to attach to the upper surface of the humerus. Used for the upstroke during flight. Enhanced skeletal rigidity: essential for flight control and maneuverability; it is apparent in the synsacrum, tarsometatarsus, and pygostyle (reduced tail). Furcula (wishbone): Flexible wishbone stores energy from the downstroke to assist with the upstroke. Keeled sternum: attachment point for the large flight muscles (pectoralis accomplishes the downstroke; the supracoracoideus the upstroke). Three-and-one feet: three toes in front, one turned to the rear (as in most living birds) allows for perching, landing, hunting, hanging, etc. Pneumatic skeleton: air sacs in bones take the place of marrow and facilitate the unique, bellows-like avian respiratory system. Bladderless urinary tract: nothing is retained that is unnecessary and might inhibit flight A key point of bird evolution. The traits above did not evolve all at once. They accumulated over millions of years and in many taxa leading from theropod B. Archaeopteryx lithographica Significance: the oldest, uncontested bird discovered to date and a real ‘missing link’, with a combination of bird-like and dinosaur-like features. Perhaps the most famous single fossil known. History: First Archaeopteryx was a single feather, found in 1860. 2 The most famous Archaeopteryx is an almost complete skeleton that is fully feathered. Thomas Huxley studied Archaeopteryx and used them to argue the case for evolution; Darwin, oddly ignored Archaeopteryx in his multiple new editions of The Origin of Species that post-dated discovery of Archaeopteryx. Only 7 skeletons/partial Archaeopteryx skeletons have been found, and all come from a fine-grained limestone of Late Jurassic age, exposed near the town of Solnhofen in Bavaria, Germany. Archaeopteryx was about the size of a modern magpie or raven. This limestone was deposited in a quiet coastal lagoon and contains well preserved remains of many animals (crabs, pterosaurs, shrimp, etc.). Archaeopteryx possesses many bird-like characters, including: expanded braincase, asymmetric feathers, keeled sternum, pneumatic bones?, three-and-one foot with sharp claws (suggesting that it flew primarily and was not a ground-runner). Not clear if big-toe was in opposition to hand. Archaeopteryx possesses many dinosaur-like characters, including: teeth, three fingered hands with claws, long tail stiffened with zygopophyses, a raptor-style saurischian pelvis, ascending process on astragalus. To be clear, a furcula that is not flexible is actually a theropod dinosaur trait. C. Other Mesozoic birds The fossil record for all birds is quite poor because their bones are delicate. We have a record thanks to exceptional preservation a some select localities. The early evolution of birds shows a gradual accumulation of flight- specialization characters in some lineages during the Cretaceous. Rahonavis: the most unspecialized Mesozoic bird excepting Archaeopteryx, yet it is of Late Cretaceous age! This bird had a long tail and an enlarged 2nd toe claw (like a dromaeosaur). It may also have a pneumatic skeleton. Post-Archaeopteryx bird evolution Stage 1: Pygostylia: acquisition of a pygostyle (Confuciusornis) Post-Archaeopteryx bird evolution Stage 2: Ornithothoraces carpometacarpus, flexible furcula, backstroke pulley in shoulder, reduced number of trunk vertebrae 3 Two Key Groups: Enantiornithes: dominant birds of Mesozoic (e.g., Iberomesornis), a few shared derived characters we won’t worry about Post-Archaeopteryx bird evolution Stage 3: Ornithurines: even shorter trunk, full caudal rotation of pubis, synsacrum, tarsometatarsus (e.g., Late Cretaceous Hesperornis (diver) and Ichthyornis (gull-like)). Alvarezesauridae: Odd flightless birds of the Late Cretaceous, including Shuuvia and Mononychus. We have no idea what to make of them. Post-Archaeopteryx bird evolution Stage 4: Aves (Neornithes): loss of teeth D. Feathered dinosaurs First discovered in the Liaoning province of China in the mid-1990’s, in lake bed sedimentary rocks approximately 124 million years old (Early Cretaceous) Include: Sinosauropteryx (1997) - a little bipedal theropod with down along its back and many sharp teeth; one specimen was found with mammal remains in its ribcage Sinornithosaurus – a deinonychosaur with vaned feathers Caudipteryx – a little sharp-toothed oviraptosaurian with symmetrically vaned feathers on its arms and tail, short arms, and GASTROLITHS Beipiasaurus – a large (ostrich-sized) therizinosaurus with primitive feathers Protarchaeopteryx – a small theropod with feathers covering most of its body, including its arms and tail and fang-like teeth Microraptor (2003) – small deinonychosaur with asymmetrically vaned feathers on its arms and legs How do we know the feathered dinosaurs (except maybe Microraptor) didn’t fly? On most, feathers are symmetrically-vaned Relatively short arms (not long like birds) Relatively few feathers on arms and tail So, what are feathers for if not flight? Insulation Display/species recognition Insect-catching Increased speed/lift 4 E. Origin of birds Objections to birds as derived theropod dinosaurs: 1. Dinosaurs with feathers (Caudipteryx, Protarchaeopteryx) are really flightless birds that evolved dinosaurian characteristics 2. The first true feathers really appear in the Late Triassic, on the non- dinosaurian archosauromorph Longisquama. 3. The first true bird (Protoavis) appeared in the Late Triassic. 4. Archaoepteryx (a bird) appears in the Late Jurassic, whereas dromaeosaurids (the supposed closest theropod relatives of birds) don’t appear in the fossil record until the Cretaceous. Current consensus: point 1 is wrong; Longisquama doesn’t have true feathers (point 2), Protoavis is too poorly known to be compelling evidence of anything (point 3), point 4 is true and a little troubling, but certainly doesn’t prove false a dinosaur ancestry for birds. After all, the oldest monotreme is also Cretaceous, but we believe its last shared ancestor with other mammals was early Jurassic. Many shared derived characters of birds resemble those of dinosaurs: 1. Perforate acetabulum: SDC of Dinosauria 2. Opisthopubic pelvis: SDC of (some Dromaeosauridae; moreover, embryonic birds have a normal saurischian pelvis that modifies during development into a synsacrum 3. Thin-walled, hollow bones: SDC of Theropoda 4. Ornithoid-style eggshell: SDC of Theropoda lizards, turtles, crocs, sauropods and ornithischians all have a distinctive eggshell microstructure that is different from the type found in theropod and bird eggshells 5. Expanded braincases and kinetic skulls: SDC of Theropoda 6. Furcula/Wishbone: SDC of many Theropoda Many non-avian theropods have been discovered with furcula, including Gorgosaurus, Albertosaurus, Tyrannosaurus, Velociraptor, Oviraptor, Segisaurus, Allosaurus, Protarchaeopteryx, Coelophysis, etc. 7. Three-fingered hand:
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